Electrodynamic transducer



April 15, 1941. H, LAUFFER 2,238,741

ned Jan, 27, 1939 Patented Apr. 15, 1941 ELECTRODYNAMIC TRANSD'UCER HansLauier, Berlin-Charlottenburg, Germany,

assignor to General Electric Company, a corporation of New YorkApplication `anuary 27, 1939, Serial No. 253,097 In Germany February 22,1938 3 Claims.

This invention is concerned with an electroacoustic transducer of thedynamic type, more particularly a microphone. What is mostly involved inapparatus of this kind is to find an acoustic oscillation system bysuitable combination of masses, elasticities and frictional resistanceswhose impedance will be constant and stable for the transmission rangeor band dealt with. However, the fulfillment of these condition isattended with difficulties whenever lowtuned oscillatory systems areconcerned which are to possess high sensitiveness or responsiveness.These diiiiculties, on the one hand, are due to disturbing or strayoscillations arising in the oscillatory system itself, while, on theother hand, they are ascribable to the air column oscillating in unisonand causing resonance, and these, in turn, cause undesirable resonancepoints in the frequency characteristic.

According to this invention, increased sensitiveness and an equalizedand smooth frequency characteristic are insured by providing ways andmeans whereby the air column outside the plunger or moving coil andwhich is formed by the exterior marginal zone of the diaphragm and themagnet system is in direct pressure equalization relationship with theoutside air, preferably through air channels or ducts, and thatseparately therefrom pressure equalization of the air column confined bythe coil and the core through the magnet gap by Way of the spaceconfined between the core `and the magnet shell which is lled withdamping means, does not take place without friction with the outside.

An exemplified embodiment of the invention is illustrated in thedrawing. Referring to the same, Fig. 1 is a section taken on line I-I ofFig. 2 through a dynamic microphone according to the present invention,while Fig. 2 is a section taken on line II-II of Fig. 1.

The magnetic eld of the dynamic microphone consists of an outer shell l,a core 2 and a base and a cover plate indicated at 3 and at 4,respectively. Base plate 3 retains the core 2 in its position inside theshell, while the free end of the core extends into a recess of the coveror top plate 4, so that an air-gap 5 is formed therewith for the passageof the magnetic field. rIhe magnetic field may be set up by means of apermanent magnet or else electrically by a winding or solenoid.

Upon the cover plate 4 the diaphragm B having centrally a preferablydome-like curvature 8 is firmly clamped tensionally at 1 by somesuitable ways and means. The diaphragm 6 by preference is tuned to a lownatural frequency compared with the frequency band to be transmitted,say, c.p.s. or even lower, by providing it with a shallow depression orrecess 9 extending or bulged in a direction contrary to the curvature ofthe dome part 8 and being chosen sufficiently thin. According to thesize of the radius of curvature of the said depression, the elasticityof the diaphragm and thus the natural pitch can be altered, the naturalfrequency becoming lower as the radius of curvature grows. Theconstruction of the diaphragm does not form a part of the presentinvention but is described and claimed in the copending application,Ser. No. D16,111 assigned to the same assignee as this application.

Inside the dome-shaped curvature 8 of the diaphragm 6 is the plungercoil I0 which protrudes into the air-gap 5 of the magnetic field. Now,the said coil l0, contradistinct to the conventional forms ofconstruction, is so designed that the surface of the diaphragm limitedby the plunger coil occupies only a part of the aggregate area of' thediaphragm. In fact, the diameter of the cylindrical coil IU maypreferably be made equal to the radius of the diaphragm 6. Such securingof the coil in the dome 8 further increases the stiffness and preventsdistortions or deformation and warping of the diaphragm 6.Contradistinct to the arrangements known in the prior art of diaphragms,in an arrangement of the plunger coil as stated, the outer marginal Zonel2 of the said diaphragm 6 also participates in the oscillations. Now,in order that a smooth and equalized frequency characteristic may besecured according to the invention, care is taken so that the air columnwhich is confined outside the plunger coil, the rim zone of thediaphragm and the cover plate 4 of the magnet system, and whichinherently would set up a marked restoring force or retractility with aresulting reduction of the amplitude of the diaphragm, is in directpressure equalizing relation with the outside air or ambient. For thispurpose, openings I I are provided in the cover plate or lid 4. Forconnection with the outside atmosphere, the magnet shell preferably isprovided with ring-sector-shaped recesses I3,

Now, to the end of precluding the production of an acousticshort-circuit it has been found to be important to secure a clear-cutseparation between the said air column and the air column confinedbetween the core 2 and the coil lll which also communicates by way ofthe air-gap 5 with the space formed by the core 2 and the magnet shellI. Such separation consists preferably of partition walls I4 mounted inthe recesses I3, said separators consisting preferably of cardboard orthe like, and coming to be placed in front of the openings II andconnecting the air column enclosed by the marginal zone I2 of thediaphragm 6, of the coil I and the cover plate 4 for pressureequalization with the outside atmosphere. The said partitions orseparators I4 have preferably an upwardly bent edge I so that thereresults a box-shaped container being open on one side and on top. Thiscontainer is so secured in the recesses I3 that its open side faces theoutside, While by the aid of its narrow longitudinal walls it ispossible to insure satisfactory attachment in the recesses I3.

So far as the air column formed by the coil I0 and the core 2 whichthrough the air-gap 5 together' with the space between the magnet shellI and the core 2 communicates with the outside air through the recessesI3 in the magnet shell, it is important that the resonance rises causedby these various resonators should be flattened down in order that alsoin this case an equallzed frequency characteristic may be obtained. Tothis end, the space conned between the core l and the magnet shell Ibelow the plunger coil Ill is preferably lled with sound deadeningmeansv I6, such as cotton or a similar fibrous material.

The air column communicating With the outside air through the opening IIin the cover plate 4 may be thrcttled while pressure equalization takesplace by choosing suitable dimensions for the diameter of the opening,with a resultant pressure drop. In this manner it is possible to insureregulation or control action, especially of the low-pitched notes.

I claim:

l. A dynamic sound transducer comprising an inner core memberconstituting the inner pole of the transducer, an outer member spacedfrom and surrounding the core member, a pole plate attached to the outermember and constituting the outer pole of the transducer, said platehaving a central opening through which the inner core member protrudesto form therewith an air gap, a diaphragm and means for rigidlysupporting its outer peripheral edge to the outer pole plate, a voicecoil immersed within the air core and attached to the diaphragm at apoint intermediate its axis and peripheral edge, said voice coil forminga pair of substantially conned air pockets, on-e between the innersurface of the voice coil, the central portion of the diaphragm and theinner core member and the other between the outer surface of the voicecoil, the outer annular portion of the diaphragm and the pole plate, andmeans whereby said confined air pockets are caused to separatelycommunicate with the air externally of the transducer, said lastmentioned'means comprising one or more apertures formed in the poleplate and a corresponding number of apertures in the outer membersurrounding the core member, and a partition member arranged anddisposed between each pair of corresponding apertures for preventing theinterchange of the air from the respective air pockets to the spaceexternally of the transducer.

2. A dynamic sound transducer according to claim l wherein the spacebetween the inner pole and the surrounding outer member is lled with asound deadening substance such as cotton or similar brous material.

3. A dynamic sound transducer according to claim l wherein the spacebetween the inner pole and the surrounding outer member and theapertures in said outer member are filled with a sound deadeningsubstance such as cotton or similar fibrous material.

HANS LAUFFER.

